Continuous ink jet recording has several advantages in that it produces low noise and can obtain a high-resolution recorded image at a high speed by utilizing a highly integrated head. Such an ink jet recording method uses as ink a solution obtained by dissolving one of various water-soluble dyes in water or a solvent mixture of water and an organic solvent. Current state-of-the-art aqueous ink jet inks utilize water soluble dyes to attain acceptable redissolvability of the dried ink on the orifice plate. This redissolvability is essential for good machine runnability as well as start-up for continuous ink jet printers which do not have a separate printhead cleaning fluid mechanism.
Because the water-soluble dye exhibits poor light resistance, when such a water-soluble dye is used, the light resistance of the recorded image frequently becomes a problem. In addition, because the ink is soluble in water, the water resistance of the recorded image also frequently becomes a problem. Namely, when rain, sweat or drinking water is applied to the recorded image, the recorded image is blurred with the ink or even disappears in some cases.
A water based pigment ink on the other hand, may be able to provide water fastness as pigments are dispersed rather than dissolved. Because writing utensils such as ball point pens, which use dyes, encounter the same problem, various kinds of aqueous pigment ink for writing utensils have been proposed for resolving the problems with respect to light resistance and water resistance. When conventional aqueous pigment ink for writing utensils is used in an ink jet printing device of the type in which printing is accomplished by discharging an ink from one or more orifices of a printhead as a stream of ink droplets, however, there is a problem in that the stability of drop stream generation is significantly deteriorated, and printing failure occurs. For pigment inks, then, redispersion, rather than redissolvability, must be achieved to have good system start-up. Use of pigments rather than dyes creates some further complications which are not present with dyes. For example, dye based inks provide films which have high dried film resistivity. Pigment based inks, particularly black inks, on the other hand, provide films which are electrically conductive.
When an ink which uses carbon black is used in ink jet printing, although the fastness of the image printed on plain paper is significantly improved, as compared with that of the image printed by using dye ink, later research reveals a disadvantage in that the density of the printed image is inferior to that of the image printed by using dye ink. Unfortunately, if the pigment concentration is increased to improve the print darkness, the discharging stability significantly deteriorated.
Because printing ink can be used at an extremely high concentration, compared to ink used in an ink jet printing device, the print density can be easily increased by increasing the pigment concentration in the ink. The density can also be increased by increasing the thickness of the coated ink layer. However, printing very dark images with ink jet printing is very difficult, because the size of the orifice is very small in high resolution ink jet printers such as Scitex continuous ink jet printers manufactured by Scitex Digital Printing, Inc., of Dayton, Ohio, and the amount of the ink printed is limited. In addition, as described above, an increase in the concentration of the pigment contained in the ink causes the occurrence of undesirable troubles in the formation of stable, straight jets. Although some of the conventional aqueous pigment inks have excellent jet forming properties for a relatively short time, such inks have a problem because the dried films of inks are quite conductive. This means that when a continuous ink jet printhead is shut down, the films formed from the dried inks tend to short out electrodes in the system, when an attempt is made to restart the printhead. On the other hand, it has been observed that inks whose dried film resistivity is too high give rise to another problem. When a jet is in "catch," that is, the drops are being recirculated rather than used for printing, there is typically an impact line on the catcher surface where the jet impacts. Just above the impact line, dried ink tends to collect, and if the ink is a pigment ink, the dried ink contains concentrated pigment. If the dried pigment is too insulating, the charge on the catch drops tends to accumulate on the area of dried pigment to the extent that it can deflect the ink stream away from the catcher. In this case, the stream of catch drops can actually reach the print media, causing an improper black streak.
Most of the traditional dispersions are made via physical grinding method using a dispersant with a pigment. Such pigment dispersions have been used for the last several years in coatings and printing industry as well as more recently in ink jet inks. More recently, a new chemically modified black dispersions have been prepared. U.S. Pat. No. 5,609,671 describes a dispersion prepared in such a manner. Due to this chemical modification, the pigment becomes less conductive compared to the unmodified particle. Even so, the conductivity is still high enough to cause charge lead to charge lead shorts in continuous ink jet printers.
It is seen then that there is a need for an improved pigment based ink suitable for use in continuous ink jet printers, which overcomes the problems associated with prior art inks. It would be particularly desirable to have such an improved pigment based ink, particularly inks based on chemically modified carbon black dispersion, suitable for achieving a dried ink film resistivity sufficient to eliminate charge lead to charge lead shorts.